I am a novice to the technicalities of the 5.1 surround format so please ignore my ignorance on the topic even if the question sounds completely stupid.

I have recently purchased a 5.1 home theatre system to go with my laptop. I wanted to know that while watching a streaming movie or video online (like on youtube), do we get the audio in the 5.1 format or is it just normal stereo? Although the vocal audio seems to come from all the speakers instead of only the front and center speakers, certain sounds which are produced in the background in a movie seems to be heard only from the rear speakers just like it is in a Dolby Digital or DTS theatres. I have read on various websites that this kind of audio is possible only through DVDs encoded in 5.1 format and there was recent news that netflix is planning to provide Dolby Digital sound in online streaming movies. So if I assume that this doesn't exist currently, then why is there a distinction in the different sounds that I hear only from the rear speakers while watching movies online?

Also do movies and videos downloaded from the net include the Dolby Digital encoding or is it possible only through DVDs purchased from stores?

depends really on what the content includes. It's cetainly possible to have 5.1 Dolby in a streamed video, I just don't know of any services that do it (if youtube allows it these days, it's extremely rare). Any video that has a 5.1 sound track should play as expected, including digitally distributed versions of DVD movies (as long as they didn't remove the 5.1 sound). It all just comes down to what the source material has in it; almost all modern video formats these days support some form of 5.1.

Youtube will encode to stereo regardless of the uploaded content. You need to review each company's policy with regard to audio encoding. For example, I know that vudu offers hd and full 5.1 surround audio, whereas, netflix currently does not (stereo only).

FYI, there are formats created by both DTS and Dolby Labs which will take stereo content and allocate the audio into a 5.1 setup using various techniques. I prefer DTS's implementation (called Neo:6) over DL's (Pro Logic II and its variants). Read your receiver's manual to understand your options.

Yeah, if you're hearing the voice come from all speakers, and your rears active (which OS, browser, player, etc.?) then probably you have DPLII or Neo:6 active in your receiver. Netflix is apparently beginning to stream in actual 5.1 but only on the PS3 so far, AFAIK.